The 75%

An exploration of Arthropods

The 75%

An exploration of Arthropods

Objectives

Students will be learning about the external and internal anatomy of arthropods.

Students will focus on the organs, structures, and functions of the integumentary system.

Students will also understand the ecological role of arthropods.

Phylum Arthropoda: The next stage in evolution

Arthropods are ancient and diverse creatures. They range from only millimeters in length to giant, bird-eating predators and phylum. This diversity is a result of 500 million years of evolution since the very first arthropod evolved and began the ancient phylum. To put that into perspective, mammals are only about 200 million years old; that's a difference of 300 million years. It's no small wonder then that arthropods make up 75% of all animals on Earth. That means that phylum Arthropoda alone has more members than all other phyla in Kingdom Animalia combined. Despite their various differences and millions of years of divergent evolution, all arthropods share a few key characteristics: bilateral symmetry, a segmented body, a chitinous exoskleton, jointed legs, and at least three pairs of legs. All of these characteristics represent a massive leap in evolution from the phyla that evolved before them. As far as reproduction, almost all arthropods lay many eggs which hatch into young, often called larvae. Since the phylum is so varied, it is difficult to make any generalizations about the life cycle of an arthropod beyond that. For the sake of efficiency, we will be focusing on two members of phylum Arthropoda in this virtual dissection: crayfish and grasshoppers.

Predators

Arthropods are eaten by almost every other creature on the planet, but it depends on the size and class of the arthropod as to what exactly feeds upon it. Everything from birds to amphibians to reptiles to fish use arthropods as energy source, because arthropods are found just about everywhere.

Habitat

Arthropods have evolved to survive in every environment imaginable, from oceans to lakes to forests, all across the world. They are truly a resilient phylum.

Eating habits

Arthropods eat almost everything. Depending on the size of the arthropod in question, it could feed on detritus, mussels, algae, fungi, plants, or even other arthropods. In the case of the grasshopper, it feeds on a diet of grass and leaves, while the crayfish feeds on small fish and other aquatic organisms.

Predators

Arthropods are eaten by almost every other creature on the planet, but it depends on the size and class of the arthropod as to what exactly feeds upon it. Everything from birds to amphibians to reptiles to fish use arthropods as energy source, because arthropods are found just about everywhere.

Habitat

Arthropods have evolved to survive in every environment imaginable, from oceans to lakes to forests, all across the world. They are truly a resilient phylum.

Eating habits

Arthropods eat almost everything. Depending on the size of the arthropod in question, it could feed on detritus, mussels, algae, fungi, plants, or even other arthropods. In the case of the grasshopper, it feeds on a diet of grass and leaves, while the crayfish feeds on small fish and other aquatic organisms.

Ecological Niche

Arthropods are so varied and widespread, that they occupy almost every ecological niche. Some are decomposers, some are primary consumers, some are secondary consumers, and some are even tertiary consumers. Oftentimes arthropods serve as a food source for other arthropods, as described above. To return to the grasshopper, it functions as a primary consumer for the most part. The crayfish is an omnivore, and so it functions as a primary, secondary, and in some cases tertiary consumer in its environment. Refer to the image on the right for a more detailed idea of the function of arthropods in their ecosystems.

Dissection

Labelled Guide

Observeall of the internal organs of the crayfish. This is to be used as a reference for identifying structures in the following pictures.

Crayfish

See the multitude of jointed limbs? This is what gives the arthropod its name, because in Latin "arthro" means jointed and "pod means foot. The large protrusions from the head are the antennae and antennules that function as sensory organs. The crayfish lives underwater, so it possesses gills as a means of respiration, though all of its internal organs are covered here by the tough exoskeleton.

Grasshopper

The grasshopper possesses much of the same features as the crayfish, only it is smaller and does not feature the prominent anterior limbs that the crayfish does. Also, since the grasshopper is a land animal, it possesses spiracles rather than gills.

Labelled Guide

Observeall of the internal organs of the crayfish. This is to be used as a reference for identifying structures in the following pictures.

Crayfish

See the multitude of jointed limbs? This is what gives the arthropod its name, because in Latin "arthro" means jointed and "pod means foot. The large protrusions from the head are the antennae and antennules that function as sensory organs. The crayfish lives underwater, so it possesses gills as a means of respiration, though all of its internal organs are covered here by the tough exoskeleton.

Grasshopper

The grasshopper possesses much of the same features as the crayfish, only it is smaller and does not feature the prominent anterior limbs that the crayfish does. Also, since the grasshopper is a land animal, it possesses spiracles rather than gills.

Highlight: The integumentary system

The integumentary system of arthropods is vastly different from that of humans. For one thing, arthropods possess an exoskeleton that serves them like armor. However, this means that arthropods cannot grow nearly as large as animals with an endoskeleton, due to the added weight and hinderance of the exoskeleton, so it's really a trade off. The exoskeleton lies on a membrane that secretes a resilient water proof layer called a cuticle. Most of the cuticle is made up of strands of tough chitin embedded in proteins (this is the exoskeleton itself). Arthropods are also segmented, which allows them to move with the exoskeleton since the overlapping segments can slide over each other to allow for movement. All of the glands that one finds in the human skin are absent once again due to the exoskeleton, but arthropods have evolved other ways to maintain homeostasis. In essence, the integumentary system of arthropods is very simplistic, but at the same time it is extremely useful